1. Field of the Invention
The present invention generally relates to soldering fluxes and, more particularly, to the fabrication, rework or repair of electronic devices, modular circuits and circuit boards and other devices including solder connections made with water soluble fluxes.
2. Description of the Prior Art
For many applications involving the joining of two or more pieces of metal to form robust electrical and mechanical connections, the formation of solder joints has been the technique of choice for many years. Soldering is especially well-suited to mounting of electrical components such as discrete elements, transistors and integrated circuit chips to circuit boards, modular packages and other substrates since very small and closely spaced connections can be made and/or reworked within relatively stringent heat budgets of the components, substrate structures or both. A flux is commonly used in conjunction with the solder to aid the flow of solder, remove oxidation on metal surfaces within the solder connection, and/or clean the surfaces of the metal substrates being joined by the solder.
In order to form such small and closely spaced connections consistent with current integrated circuit and packaging technologies, soldering techniques and materials have become highly developed and sophisticated in recent years. It is now common for fabrication and rework procedures for electronic components and other complex metal structures to involve numerous steps with slightly different solder alloys and fluxes employed during each step. Soldering conditions thus become quite critical to differentially affect the differing solder and flux materials, and complex testing and inspection procedures are necessary to assure that soldering operations have been correctly carried out.
One particularly difficult problem is presented by residual flux which may remain near a solder connection or elsewhere on the device. When it is considered that one of the functions of a flux is to remove oxidation on metal surfaces within the solder connection, it can be understood that the potential for chemical reaction or corrosion of other metal surfaces is presented by residual flux. Accordingly, for some critical applications, so-called "no clean" fluxes have been developed which do not cause corrosion and residual flux may often be left on the device. However, where residual flux must be removed as an incident of the design (e.g. because of the particular metals to be joined) or to allow rework of the device (since the presence of flux may cause undesired solder deposit or flow during such procedures), water soluble fluxes must be used to allow flux removal.
Water soluble fluxes, regardless of composition, may be more "aggressive" than "no clean" fluxes and are known to cause corrosion of most metals which are desirable for electronic component manufacture. Even though it is common to vigorously wash components following manufacturing processes with a flow of high temperature (e.g. 140.degree. F. (60.degree. C.)) water, residual water soluble fluxes may not be completely removed and resultant corrosion can cause failure of a component long after it is placed in service. For that reason, detection of residual flux is not susceptible to electrical testing. Chemical testing for residual flux is difficult, time-consuming, expensive and critical, yielding false-positives and failing to detect residual flux in a significant number of tests while being economically impractical to apply to more than a small sample of manufactured devices.
Inspection is also difficult in view of problems of visual access (e.g. where a component may extend over a location where residual flux may be deposited) and may be further complicated in the presence of "no clean" fluxes which are not readily distinguishable in appearance from water soluble fluxes and the fact that it may be desirable, in some cases, to leave residual "no clean" fluxes in place, for example, to prevent oxidation of the solder connection surface. Thus, upon inspection after washing, it is often assumed that any remaining flux is of the "no clean" type.
Therefore, it can be understood that no available inspection and testing technique or combination thereof is fully adequate to avoid corrosion due to residual water soluble flux after completion of manufacture and consequent compromise of reliability of the manufactured device.
Additionally, photo active dyes are known which, when irradiated with energy of a particular wavelength, will absorb photons at that wavelength and re-radiate photons at another lower (longer) wavelength. Such dyes are generally referred to collectively as fluorescent dyes. However, these materials also present particular difficulties for use with a water soluble solder flux.
Importantly, to be suitable for distinguishing or detecting a particular water soluble flux, the fluorescent dye must be water soluble so that it can be removed with the water soluble flux in order to provide any utility for indicating the presence of residual flux remaining after washing. Moreover, solubility of the dye must be somewhat closely matched to the water solubility of the flux so that, on the one hand, false positive inspections (as would be expected from lower dye solubility) would be very rare and, on the other hand, so that failure of detection is prevented if, for example, higher water solubility allowed the dye to be leached from the flux during washing. Most conventional fluorescent dyes are known to be insoluble or of very limited solubility in water and therefore completely unsuited for the intended application to water soluble fluxes.
Moreover, virtually all conventional fluorescent dyes are acidic while many water soluble fluxes, especially those appropriate to electronic device manufacturing are moderately basic in pH. Therefore, known fluorescent dyes are very likely to react with the materials in a water soluble solder flux and change the properties of the flux. Even if the dye is relatively inert and the reaction is slow, shelf life of the flux would be at least compromised and the likelihood of production of unreliable solder connections would increase, resulting in reduced manufacturing yield.
Further, a problem unappreciated prior to the present invention was that even certain conventional dyes considered to be substantially "inert" (i.e., non-chemically reactive with the flux) do, in fact, produce observable reactions at relatively low concentrations in water soluble solder fluxes. Reduction of concentration of fluorescent dye in a flux, however, limits the amount of energy which can be re-radiated from residual flux and would thus be expected to compromise or severely limit the ability of such a material to improve an optical inspection process.
As noted above, solder fluxes suitable for electronic devices are highly caustic while fluorescent dyes are generally acidic. Thus, even if a dye was actually inert, the mixture of an acid dye with a caustic flux will cause the mixture to become more acidic (or less alkaline) with the amount of dye added. Therefore there would be a further trade-off between compromise of the function of the flux to remove surface contamination and promote solderability and the amount of dye available for re-radiation of energy during optical testing. Further, any acidity of the dye could, itself, cause etching of metals adjacent the solder connection and compromise manufacturing yield and/or device reliability. Also, known fluorescent dyes degrade when exposed to elevated temperatures. In fact, it appears that the use of dyes in solder fluxes has been largely limited to temperature indicators as disclosed in U.S. Pat. Nos. 4,505,421, 4,563,224, 4,688,713 and 4,809,901. In these patents a dye is added to the solder flux and heat is applied until an irreversible color change of the flux is observed when the flux containing the dye reaches a critical temperature corresponding to a desired state of the solder. The dye is rendered unavailable for any future re-use.